Many electrical systems incorporate one or more large electrical motors operative to move heavy loads in environments in which extreme precision of motor control is required. Often, a brush-type DC motor is utilized to provide the power for such systems. Examples of systems requiring large high-power, precision controlled operation include locking radar antenna, rotating weapons platforms, horizontal continuous casting, printing machinery and heavy equipment assembly line operations. In such systems, the control of the brush DC motor to the precision required by the application is often difficult to achieve in view of the high currents which the system utilizes and the precision of control coupled with the heavy inertia generally found in the system loads. To meet the control requirements of such systems, practitioners in the art have created a family of devices often referred to as brush motor controllers or servo motor amplifiers. Such control amplifiers provide the basic function of precision control of the large DC armature currents present in the brush motors, usually under the control of a low voltage control system, which itself may be computerized and extremely sophisticated. In the majority of applications of the foregoing described systems, there frequently exists circumstances in the operative environment which require that certain operational limits be placed upon the motor system to protect against potential system failures.
In brush motor amplifier systems in which the angular position of the motor armature must necessarily be determined accurately, a digital encoder is often mechanically coupled to the armature of the motor and provides a digitized signal input to the electronic control system used to operate the brush motor amplifier and the brush motor. Frequently, a computerized system is operative in response to the digitized output of the encoder to provide the various control and error signals necessary to operate the brush motor in accordance with the desired system motion. For example, in the horizontal continuous casting art in which an elongated billet is continuously formed and moved along a casting bed in accordance with a predetermined motion profile, a plurality of individually controlled drive motors are operative upon the casting. In the case of horizontal continuous casting, it is extremely important that the physical position of the billet be communicated to the casting control system at all times. By way of further example, in the use of locking radar antenna, it is extremely important that the rotational position of the radar antenna be accurately communicated to the computing and display portions of the radar system. Many other systems using such high power brush DC motors impose a similar requirement for precise determination of the angular position of the motor armature.
A problem arises in many encoder driven systems in that the system precision mandates that the encoder be capable of resolving the armature position to a very small increment. On the other hand, where the system requires relatively higher speed motion of the armature of the motor, there arises a potential that the system will erroneously interpret the higher speed of the encoder as a continuous line rather than a succession of high speed passing segments. In such case, a failure mode is entered, often at high consequence, in which the system perceives that the load is static rather than moving and increases current to the motor. The failure mode, once entered, continues to increase unchecked until some physical limit of the system is reached.
In systems utilizing other means of determining the position of the armature, the basic system potential for failure often continues to exist. This potential arises from the fact that characteristics utilized to improve system resolution generally raise the possibility that a similar system error to that described above for encoders may occur. In essence, the high resolution of the system makes it vulnerable to misinterpretation of incoming data at higher system speeds.
There exists therefore a need in the art for an improved brush motor control system which avoids the foregoing described overspeed failure mechanism.